Arctic sea ice reached a record low in September 2007, below the previous record set in 2005 and substantially below the long-term average. This image shows the Arctic as observed by the Advanced Microwave Scanning Radiometer for EOS (AMSR-E) aboard NASA’s Aqua satellite on September 16, 2007. In this image, blue indicates open water, white indicates high sea ice concentration, and turquoise indicates loosely packed sea ice. The black circle at the North Pole results from an absence of data as the satellite does not make observations that far north.

Three contour lines appear on this image. The red line is the 2007 minimum, as of September 15, about the same time the record low was reached, and it almost exactly fits the sea ice observed by AMSR-E. The green line indicates the 2005 minimum, the previous record low. The yellow line indicates the median minimum from 1979 to 2000.

The contour lines in this image show sea ice extent, as reported by the National Snow and Ice Data Center (NSIDC). Another measure of sea ice is area, and this value was initially reported as a record low by The Cryosphere Today at the University of Illinois. A simple analogy for these terms is a slice of Swiss cheese. Extent counts everything inside the slice’s perimeter as cheese-filled whereas area subtracts the holes from the total amount.

A more technical explanation of area versus extent involves pixels. A pixel is the smallest possible unit of the satellite image. (How much of the planet’s surface a pixel covers depends on the satellite sensor.) Measurements of sea ice area total the amount of sea ice in each pixel. Extent, as measured by NSIDC, sets a threshold of 15 percent, and counts any pixel above that threshold as completely ice-filled. Consequently, estimates of sea ice extent are higher than estimates of sea ice area.

NASA image created by Jesse Allen, using AMSR-E data courtesy of the National Snow and Ice Data Center (NSIDC), and sea ice extent contours courtesy of Terry Haran and Matt Savoie, NSIDC, based on Special Sensor Microwave Imager (SSM/I) data.